Process for controlling the pressure and delivery of a fuel feed to a servovalve unit

ABSTRACT

A device for controlling fuel pressure and delivery as necessary for the operation of the servovalves of an aircraft engine is disposed downstream of a fuel injector feed unit of the engine. The device comprises a chamber maintained at the servovalve feed pressure by the movement of a closure member to vary the opening of a port of the chamber. The closure member moves under the action of opposing forces comprising inter alia a force in the opening direction proportional to the servovalve feed pressure. An equilibrium position of the closure member is a function of pressure resulting from the engine speed. An injector delivery control loop is therefore isolated from a servovalve pressure and delivery control loop.

This application is a Division of application Ser. No. 09/805,036 filedon Mar. 14, 2001.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to hydromechanical devices for meteringthe delivery and pressure of a fuel feed to a hydraulic servovalve unitwhich is driven by the fuel. The invention also relates to a fuel supplycircuit comprising said device and to a process for controlling thepressure and delivery of the fuel to a servovalve unit.

2. Technological Background

In an aircraft engine the fuel is supplied to fuel injectors at whichthe fuel is burned to provide the propulsive power and the power forfeeding engine auxiliaries. The fuel is also used as a cooling liquid inheat exchangers, and also to operate a plurality of hydraulicservovalves. For example, nozzles and other variable-geometry elementsare operated by hydraulic rams which are driven by a supply of the fuelunder pressure.

The servovalves are usually supplied from a central servovalve feed unitwhose pressure must remain constant and whose delivery must be capableof being varied in accordance with servovalve requirements.

The supply circuit for the injectors, exchangers and central servovalvefeed unit comprises a low pressure pump which raises fuel coming fromthe aircraft tanks at an initial pressure Pca to a pressure Pb, a highpressure pump which further raises the fuel to a pressure higher thanthat required for the injectors and the servovalve feed unit. Unusedfuel is recycled. The proportion of recycled fuel relative to consumedfuel must not be excessive since the fuel is heated by its passagethrough the high pressure pump and is therefore less suitable as acooling liquid. The high pressure pump may be a pump whose delivery isdetermined solely by its speed of rotation. This speed is in turn afunction of engine speed. The pump may also be a pump whose delivery isa function not only of its speed but also of another parameter which canbe so controlled that a pump running at a given speed can providedifferent delivery rates.

As explained in U.S. Pat. No. 5,715,674 to Reuter, variable deliverypumps have the advantage over fixed pumps of being able to deliver afuel flow rate theoretically adapted to the instantaneous deliveryrequired for engine operation.

The term “theoretically” is used because, as explained in the saidReuter patent, the pump response time is not negligible. Moreparticularly, the servovalves introduce erratic delivery variationsrequiring a control bandwidth at frequencies incompatible with pumpcontrol capacities whether the pump is of the variable geometry kind orof the fixed delivery kind. Excessive response times may lead todecrease in deliveries to the engine fuel injectors and/or malfunctionof the servomechanisms operated by the pressurised fuel.

To overcome this problem the Reuter patent provides a pump control valvehaving an outlet connected to an element for controlling the delivery ofthe pump. The operation of the valve is not explained and, inparticular, it is not clear how the pump can provide a better transientresponse time as stated in the last two lines of column 6 and the firstline of column 7 of Reuter. It is clear, however, that the valve is anessential element of a control loop aimed at reducing as much aspossible the delivery of the pump. This delivery is theoreticallymaintained at a level slightly above the level necessary to cover thedelivery demanded by the engine fuel injectors and the delivery demandedat any instant by the servovalves.

Although such a device is an improvement over the prior art it does notprovide a response time short enough to meet the requirements of theservovalves controlling various elements of the engine.

SUMMARY OF THE INVENTION

The inventors have realised that the response time of variable deliverypumps is short enough for a control loop comprising such a pump tocontrol the fuel delivery to the engine fuel injectors without majorproblems. The latter delivery varies in accordance with known laws independence upon engine speed and other parameters such as temperatureand the external pressure, which it is possible to monitor continuouslyso that they are known at any instant. Since the engine is a highinertia device its speed of rotation varies slowly. Similarconsiderations apply to the other parameters affecting the deliveryconsumed by the injectors.

The invention is therefore based on isolating the injector deliverycontrol loop from the servovalve feed control loop.

To this end, the delivery required from the high pressure pump at anyinstant needs to be the sum of the delivery required by the engine fuelinjectors at that instant plus the maximum delivery which may berequired by the servovalves.

The maximum servovalve delivery is the delivery which would be necessaryif all the servovalves were simultaneously each to demand the maximumdelivery consumable by that servovalve. The sum of the maximumdeliveries to all the servovalves for a given engine speed is thereforea known constant, so that the delivery variations of the pump are solelythe variations of the fuel injector consumption. Delivery to the fuelinjectors is therefore readily controllable.

When the required portion of the pump delivery has been delivered to thefuel injectors, the remainder of the delivery is sufficient to meet allservovalve requirements in all circumstances.

The invention therefore provides a pressure and delivery control loopfor fuel directed to a servovalve feed unit downstream of a fuelinjector feed unit, comprising a fuel intake at a first pressure and ata constant controlled delivery and a control device for keeping thepressure of the feed unit constant whatever the consumption of said unitand the variations of the various fuel pressures in dependence upon theengine speed and upon the other parameters affecting the pressures.

In an embodiment of the invention the pressure control device comprisesa chamber for providing this constant pressure. The chamber communicateswith the feed unit and, by way of a variable cross-section outlet port,with a portion of the fuel circuit between the high pressure pump andthe low pressure pump.

The cross-section of the outlet port is varied in dependence upon theposition of a first shutter which moves around a variable equilibriumposition. This position is a function of the pressure difference betweena first pressure downstream of the low pressure pump and upstream of thehigh pressure pump and a second pressure upstream of the low pressurepump. The movements of the first shutter around the variable equilibriumposition are a function of the fuel consumed by the servovalve feedunit.

The variation of the port cross-section around the variable equilibriumposition is effected, as explained above, by means of a first shuttermovable in response to two opposing forces. One of these forces isprovided by the combined action of the second pressure and of resilientmeans, such as a spring, and the other force is provided by the actionof the first pressure on the shutter.

Variation of the port cross-section around the equilibrium position independence upon feed unit consumption is effected by the movement of asecond movable shutter which is movable in response to two opposingforces. One of these forces is provided by the pressure forces exertedby the fuel combined with resilient forces created, for example, by aspring. The other force is created by fuel pressure forces. One of theseother forces is created by fuel at the pressure of the servovalve feed.Consequently, a decrease in the latter pressure reduces the latter forceand therefore leads to a closing movement of the second shutter.

For the sake of simplicity and improved reliability, the two shutters ofthe chamber are merged to form a single element formed by part of amovable spool which variably masks the opening of the outlet port. Thespool has two ends and an intermediate part forming a piston.

The spool piston divides a compartment into two half-chambers, namely afirst half-chamber and a second half-chamber, the latter being at thecontrolled servovalve feed pressure. The first half-chamber communicateswith the upstream side of a diaphragm, and the second half-chambercommunicates with the downstream side of the diaphragm. The upstreamside of the diaphragm receives fuel originating from the high pressurepump, preferably through a filter. A spring applies a pressure to thespool in a direction towards the second half-chamber.

The first end of the spool is subjected to the pressure Pca and itsopposite end is subjected to the pressure Pb.

The forces acting on the spool and tending to reduce the opening of theoutlet port are:

the force exerted by the pressure Pca acting on the first end of thespool;

the force of the spring; and

the force exerted by the pressure upstream of the diaphragm and actingon the spool piston.

The opposing forces acting on the spool and tending to increase theopening of the outlet port are:

the force exerted by the pressure Pb acting on the second end of thespool; and

the force exerted by the pressure downstream of the diaphragm and actingon the spool piston.

The parameters on which a direct action is possible by design and whichmust be considered in order to obtain a constant controlled pressure independence upon servovalve delivery are:

the cross-sectional areas of the first and second ends of the spool;

the diaphragm cross-section;

the elastic coefficient of the spring; and

the area of the spool piston.

In summary, according to the invention, in a fuel supply circuit of anaircraft engine including fuel injectors and a plurality of servovalves,a low pressure pump for raising the pressure of fuel from a tank from apressure Pca to a pressure Pb, and a high pressure pump and filter forfurther raising the pressure of the fuel to a pressure Psf at the outletof said filter, there is provided a device for controlling the pressureand delivery of fuel to said plurality of servovalves comprising:

a compartment divided in a sealed manner to define a first end chamber,an intermediate chamber, and a second end chamber;

a spool movable in said compartment and having first and second endportions and an intermediate portion;

said first end portion being engaged in a sealed manner in said firstend chamber;

said intermediate portion including a piston dividing said intermediatechamber in a fluid tight manner into first and second half-chambers;

means for communicating said first end chamber with fuel at the pressurePca;

means for communicating said second end chamber with fuel at thepressure Pb;

a first outlet port in said intermediate chamber in communication withfuel at said pressure Pb;

resilient means disposed in said first half-chamber and acting to urgesaid spool in the direction of said second end chamber;

diaphragm means having an inlet and an outlet;

said inlet being connected to receive fuel at the pressure Psf and alsobeing connected to said first half-chamber;

said outlet being connected to a fuel feed line to said servovalves andalso being connected to said second half-chamber;

said spool being movable in said compartment in response to forcesexerted on said spool by said resilient means and the fuel pressuresapplied to said first and second end chambers and to said first andsecond half-chambers to cause said piston to vary the opening of saidfirst outlet port.

Preferably, the cross-sectional areas of the first and second ends ofthe spool are identical. Consequently, the force tending to increase theoutlet port opening is proportional to the pressure difference Pb−Pca.This pressure difference is in turn proportional to the square of theengine speed. The delivery through the diaphragm is proportional to thepressure upstream thereof.

Consequently, the delivery passing through the control device andconsisting of the sum of the delivery to the servovalves and the returndelivery from the exit of the variable outlet port of the secondhalf-chamber is proportional to engine speed, at least within a certainrange.. This characteristic is well adapted to the delivery necessaryfor the servovalves, whose requirements increase with engine speed.

The invention also provides a process for controlling the pressure anddelivery of fuel to said plurality of servovalves of an aircraft engine,comprising the steps of:

controlling the high pressure pump delivery so that, whatever the enginespeed, the delivery from said high pressure pump corresponds to the sumof the delivery required by said fuel injectors at said engine speed andthe prevailing flight conditions and the delivery which would berequired by said plurality of servovalves if all of said servovalvesoperate simultaneously using the maximum delivery usable at said enginespeed;

placing a servovalve feed unit downstream of a fuel injector feed unitand in communication with a compartment having an opening which isvariable in dependence upon the position of a movable closure member;and,

causing said closure member to move to vary said opening in response toopposing first and second groups of forces, said first group of forcesacting in the closing direction and comprising a resilient force, aforce proportional to the pressure Pca, and a force proportional to afirst pressure, and said second group of forces acting in the openingdirection and comprising a force proportional to the pressure Pb, and aforce proportional to a second pressure less than the first pressure andat a substantially constant difference therefrom.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an aircraft engine fuel supply circuitincorporating an embodiment of a servovalve delivery control device inaccordance with the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The fuel supply circuit of FIG. 1 is shown in very simplified form, onlythe connections of the servovalve control device being shown, elementssuch as filters, heat exchangers and bypasses which are conventionallyused in aircraft fuel supply circuits being omitted.

The fuel arrives from an aircraft fuel tank via a line 1 at a pressurePca. Its pressure is raised by a low pressure feed pump 2 which deliversfuel at a pressure Pb. The fuel leaving the pump 2, together with fuelfrom a return line which will be referred to hereinafter, are deliveredvia a line 21 to a high pressure pump 3 whose output is adjustableaccording to the drive speed.

In accordance with the invention, the controlled delivery from the pump3 is equal to the sum of the delivery needed to supply the engine fuelinjectors and the maximum possible delivery which may be consumed by thehydromechanical devices which use the pressurized fuel as the hydraulicdriving fluid. Consequently, the delivery variations to be accommodatedby the pump 3 are merely the variations in the delivery to the enginefuel injectors. These variations are slow enough to be controlledwithout difficulty. More particularly, they are compatible with the pumpresponse time.

The fuel from the pump 3 passes through a line 22 to a self-cleaningfilter 4 which, in known manner, has an outlet 5 for unfiltered fuel andan outlet 6 for filtered fuel.

The unfiltered fuel is delivered to a unit 9 for metering fuel to theengine fuel injectors. The filtered fuel from the outlet 6 is directedthrough a heater 7 before being supplied for use by servovalvesoperating mechanisms of the engine .

In accordance with the invention a device 10 is provided to maintain aconstant servovalve feed pressure whatever delivery is taken by theservovalves which drive the engine mechanisms.

The device 10 therefore receives, via line 8, a fuel delivery which issubstantially equal to the maximum possible delivery required if all ofthe servovalves are to be operated simultaneously. The proportion of thefuel actually required by the servovalves is delivered by the deliverycontrol device 10 through a line 23 to the servovalves, and the unusedproportion is returned via a line 24 to the line 21 feeding the highpressure pump 3 as previously stated.

The servo delivery control device basically comprises a compartment 11divided into three chambers which are sealed from one another, namely atop chamber 12, an intermediate chamber 13, and a bottom chamber 14. Thewords “top” and “bottom” are used merely to differentiate the chambersfrom one another. In the actual equipment the chambers can havepositions relative to one another which are different as regards theirrespective levels. If preferred the top and bottom chambers may bereferred to as first and second chambers respectively.

In this embodiment a spool 30 is slidably mounted in the compartment 11,the spool 30 comprising a rod 31 having a shoulder 33 forming a pistonin the intermediate chamber 13. The top end 32 of the rod extendssealingly into the top chamber 12, and the bottom end 34 of the rodextends sealingly into the bottom chamber 14. The intermediate chamber13 is divided by the piston 33 into two half-chambers, namely an upperintermediate half-chamber 15 and a lower intermediate half-chamber 16.

The filtered fuel delivered to the device 10 via line 8 has a pressurePsf and is supplied to the upper half-chamber 15 and to the inlet of adiaphragm device 17. The outlet of the diaphragm device 17 communicateswith the servovalve feed line 23 and with the lower half-chamber 16though a port 18 thereof.

Fuel not used by the servovalves exits the lower half-chamber 16 througha variable cross-section outlet port 19 for recycling via lines 24 and21. The opening of the outlet port 19 is controlled by movement of thepiston 33 within the intermediate chamber 13.

A spring 35 is disposed in the upper half-chamber 15 and bears on thepiston 33 to apply thereto a force tending to move it towards the lowerhalf-chamber.

The position adopted by the piston 33 in the intermediate chamber 13 isa function of the force exerted by the spring 35, the pressure Pca ofthe fuel acting on the top end 32 of the spool 30, and the pressure Pbof the fuel acting on the bottom end 34 of the spool 30. Also, thepressure difference Pb−Pca is proportional to the square of the numberof engine revolutions since the pumps 2 and 3 rotate at speedsproportional to the number of engine revolutions per minute.

Also, the diaphragm 17 maintains a constant pressure difference betweenits inlet and outlet sides, and the position of the spool 30 is alsocontrolled by this pressure difference because the pressure Psf acts onthe top surface of the piston 33 and the pressure at the outlet of thediaphragm 17 acts on the bottom surface of the piston 33.

In the equilibrium state the pressure in the bottom chamber is equal tothe pressure necessary to feed the servovalves—i.e., the fuel deliveryarriving through the diaphragm 17 and the fuel delivery Qc escapingthrough the port 19 of the intermediate chamber 13 are such as toproduce the latter pressure.

Let us now assume that, with all other factors remaining the same, inparticular the engine speed and therefore the pressures Pb and Psf, aservovalve calls for fuel. This will cause the pressure in the lowerhalf-chamber 16 to drop, and the pressure Psf in the upper half-chamber15 and the spring 35 will urge the piston 33 downwards with the resultthat the area of the port opening 19, and therefore the flowtherethrough, is reduced until a new equilibrium state is reached. Inthe case of a pressure increase the converse phenomenon (upward movementof the piston) leads to an increase of the delivery through the port 19.

Clearly, therefore, from a variable equilibrium position dependent uponthe pressures supplied by the pumps, the spool will move to keep thepressure available for the servovalves constant whatever the servovalvedemand may be.

Preferably, safety elements are provided to ensure that possible defectsdo not lead to impaired operation. For example, an overpressure reliefvalve 20 is placed in parallel with the diaphragm 17 and will open inthe event of the diaphragm 17 becoming clogged or icing up, thuslimiting the pressure drop across the diaphragm.

A mechanical stop 26 limits the upwards travel of the spool 30—i.e.movement in a direction which increases the opening of the adjustableport 19. The stop 26 will limit the permissible return delivery at theexit of the bottom half-chamber in the event of the pressure differencePsf−Pb dropping below a minimum value. The stop 26 will thereforeguarantee a delivery margin to the engine fuel injectors between speedsranging from idling to full throttle operation, but could riot guaranteethis margin for in-flight re-ignition.

Preferably, the stop 26 is in the form of a screw introduced into atapped aperture parallel to the direction of spool movement. Theposition of the stop 26 can then be adjusted.

In an embodiment which also provides improved control of pump bearingcooling, the bottom half-chamber has a second outlet port 29 forpartially supplying the cooling exchanger of the pump bearings. The port29 is so positioned as to be closed i.e., completely masked by thepiston 33—at low engine speeds—i.e., the port 29 is completed masked bythe piston 33 when the opening of the port 19 is reduced.

When engine speed increases, the pressure Pb increases and theequilibrium position of the piston 33 rises, thus releasing a deliveryof fuel for pump bearing cooling. Suppressing the cooling delivery atlow engine speeds leads to readier optimisation of pump bearing coolingat high speeds.

We claim:
 1. In a fuel supply circuit of an aircraft engine includingfuel injectors and a plurality of servovalves, a low pressure pump forraising the pressure of fuel from a tank from a pressure Pca to apressure Pb, and a high pressure pump and filter for further raising thepressure of the fuel to a pressure Psf at the outlet of said filter, aprocess for controlling the pressure and delivery of fuel to saidplurality of servovalves comprising the steps of: controlling the highpressure pump delivery so that, whatever the engine speed, the deliveryfrom said high pressure pump corresponds to the sum of the deliveryrequired by said fuel injectors at said engine speed and the prevailingflight conditions and the delivery which would be required by saidplurality of servovalves if all of said servovalves operatesimultaneously using the maximum delivery usable at said engine speed;placing a servovalve feed unit downstream of a fuel injector feed unitand in communication with a compartment having an opening which isvariable in dependence upon the position of a movable closure member;and, causing said closure member to move to vary said opening inresponse to opposing first and second groups of forces, said first groupof forces acting in the closing direction and comprising a resilientforce, a force proportional to the pressure Pca, and a forceproportional to a first pressure, and said second group of forces actingin the opening direction and comprising a force proportional to thepressure Pb, and a force proportional to a second pressure less than thefirst pressure and at a substantially constant difference therefrom.